Crabgrass generally appears in lawns in late May, so earlier in the spring is the time for applying herbicides -- if you want to get the jump on crabgrass. You'll want to use pre-emergent herbicides, specifically....
May 1 is the approximate time for application of a pre-emergent herbicide to control crabgrass in many regions of the northern U.S. and Canada, but the ideal time will vary depending on where you live and how warm a spring you're having. In the South, you'll need to apply crabgrass preventer earlier.
Pre-emergent herbicides kill crabgrass at a specific time (as their name suggests): before its seedlings emerge. For success in killing crabgrass with pre-emergent herbicides, timing is critical, and you can use nature as your guide. The old saying is that pre-emergent herbicides should be applied sometime between the time the forsythias stop blooming and the lilacs start blooming.
Corn gluten is considered an organic alternative to using chemical pre-emergent herbicides.
